A small group of moderate Republican senators, worried that their leaders’ health-care bill could damage the nation’s social safety net, may pose at least as significant an obstacle to the measure’s passage as their colleagues on the right.

The vast changes the legislation would make to Medicaid, the country’s broadest source of public health insurance, would represent the largest single step the government has ever taken toward conservatives’ long-held goal of reining in federal spending on health-care entitlement programs in favor of a free-market system.

That dramatic shift and the bill’s bold redistribution of wealth — the billions of dollars taken from coverage for the poor would help fund tax cuts for the wealthy — is creating substantial anxiety for several Republican moderates whose states have especially benefited from the expansion of Medicaid that the Affordable Care Act has allowed since 2014.

Their concerns that the legislation would harm the nation’s most vulnerable and cause many Americans to become uninsured have thrust into stark relief the ideological fault lines within the GOP. Though Senate conservatives were the first to threaten to torpedo the bill, contending that it is too generous, the potential loss of half a dozen moderate lawmakers’ votes may be the main hurdle. Since the bill will get no support from Democrats, Senate Majority Leader Mitch McConnell can afford defections from no more than two Republicans as he tries to bring it to a vote this week.

His odds worsened Friday when Sen. Dean Heller (R-Nev.), who is up for reelection next year, said he could not support the bill in its current form. Heller specifically cited its cuts to Medicaid, not just by ending its expansion in Nevada and 30 other states but by restricting government spending for the program starting in 2025.

(Peter Stevenson/The Washington Post)

This bill “is simply not the answer,” he declared, describing some of the 200,000 Nevadans who have gained health coverage through the expansion. He rhetorically asked whether the Republican plan will ensure that they have insurance in the future. “I’m telling you, right now it doesn’t do that,” he said.

Though four of the other five wavering GOP centrists also come from Medicaid-expansion states, not all were as explicit as Heller in their reactions after the Better Care Reconciliation Act was finally unveiled late last week. Both Sens. Shelley Moore Capito (W.Va.) and Lisa Murkowski (Alaska) said that they would evaluate it with an eye toward its effect on low-income residents.

“It needs to be done right,” Murkowski said in a tweet. “I remain committed to ensuring that all Alaskans have access to affordable, quality health care.”

Louisiana Sen. Bill Cassidy, a physician, also cited concerns about how the bill would affect his state. “Right now I’m undecided,” he told Face the Nation in an interview that aired Sunday.

Part of the pressure the moderates now face is that Medicaid consistently draws widespread support in surveys. A poll released Friday by the Kaiser Family Foundation found that three-fourths of the public, including 6 in 10 Republicans, said they have a positive view of the program. Just a third of those polled said they supported the idea of reducing federal funding for the expansion or limiting how much money a state receives for all beneficiaries.

Even among Republicans, the foundation found, only about half favor reversing the federal money for Medicaid expansion.

Congressional budget analysts plan to issue their projections as early as Monday on the legislation’s impact on federal deficits and the number of Americans with insurance coverage. Already, proponents and critics alike are predicting that the Senate proposal would lead to greater reductions through the Medicaid changes than the $834 billion estimated for a similar bill passed by House Republicans last month.

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“The focus of Republican efforts largely has been on costs,” said Lanhee Chen, a research fellow at Stanford University’s Hoover Institution. “You do have a different set of issues that the two sides have been focused on, which partly explains why this has been such an intractable and difficult debate to find common ground on.”

Under the Senate GOP version, 2021 is when Medicaid’s transformation would begin. The expansion, which has provided coverage to roughly 11 million people, would be phased out. What is now an open-ended entitlement, with federal funding available for a specific share of whatever each state spends, would be converted to per capita payments or block grants.

Then, four years later, the federal government would apply an inflation factor to spending increases that would be equal to the urban consumer price index rather than the higher medical inflation rate used in the House bill.

“There has never been a rollback of basic services to Americans like this ever in U.S. history,” said Bruce Siegel, president of America’s Essential Hospitals, a coalition of about 300 hospitals that treat a large share of low-income patients. “Let’s not mince words. This bill will close hospitals. It will hammer rural hospitals, it will close nursing homes. It will lead to disabled children not getting services. . . . People will die.”

To some extent, the division within the GOP’s ranks reflects geography. Some of the most reticent senators come from states where health-care systems stand to lose the most financially if the bill passed.

According to an analysis by the Commonwealth Fund, hospitals in Nevada would be saddled over the next decade with at least double the costs in “uncompensated care” — bills for which neither an insurer nor a patient paid. It examined the House legislation but noted that the Senate bill would doubtless hit harder because of its deeper reductions in federal Medicaid payments.

Hospitals in West Virginia would suffer an even greater spike in uncompensated care, about 122 percent during the decade. But the analysis showed that the greatest damage would come in McConnell’s own state: Kentucky, which has had the nation’s largest Medicaid expansion under the ACA, would see a 165 percent jump in unpaid hospital bills.

Yet conservative Sen. Patrick J. Toomey (R-Pa.), one of the bill’s champions, said it would establish “a very, very gradual and gentle transition to a normal inflation rate” for a program in which he said costs were spiraling out of control. Beyond Medicaid, it would permit private health plans to cover fewer services and would allow individuals and employers to eschew coverage without penalty — elements that its authors say could lower how much consumers pay for their insurance.

“The idea that there’s a sector of our economy that has to permanently have a higher inflation rate than the rest of our economy is ridiculous,” Toomey said Thursday. “I think that it’s absolutely essential to putting [Medicaid] on a sustainable path so that it will be there for future generations.”

Avik Roy, a conservative health expert who serves as president of the Foundation for Research on Equal Opportunity, said the legislation’s proponents need to show “that competitive insurance markets can work for the poor and the vulnerable and the sick.”

People too often equate federal spending with establishing a safety net, when greater competition and a free market could produce better results at a lower cost, in Roy’s view. The Senate bill would extend “quite robust” tax credits to many people, he said, even to those living in poverty who were not eligible for Medicaid: “Republicans have a different view of what a safety net should look like.”

Pressure is coming from outside groups on the right. Though the four conservatives who have voiced opposition to the bill might be pushed hard — Sens. Rand Paul (Ky.), Mike Lee (Utah), Ted Cruz (Tex.) and Ron Johnson (Wis.) — Heller will be a special target. A super PAC, America First Policies, reportedly is planning a seven-figure ad buy just in Nevada.

But patient-advocacy organizations that focus on an array of diseases are intensifying their own lobbying on the bill, including running print and online ads in several key states. If one health issue has emerged as a flash point, however, it is the nation’s opioid epidemic.

Shatterproof, a national nonprofit organization focused on addressing addiction, estimates that 2.8 million people have gained access to substance-abuse treatment under Medicaid expansion. In Ohio alone, total federal funding provided 70 percent of the $939 million that the state spent to combat the epidemic last year.

Capito and Sen. Rob Portman (Ohio) have asked the chamber’s Republican leaders to provide in the bill $45 billion over 10 years to address opioids; the measure currently provides $2 billion. But that amount, Shatterproof chief executive Gary Mendell said Friday, is less than a tenth of what experts predict will be needed over the next decade. And providing a designated fund while leaving millions uninsured makes little sense, he added.

Shatterproof and the National Council on Behavioral Health just launched a six-figure advertising buy in Ohio, West Virginia and Maine — which is represented by another undecided Republican, Sen. Susan Collins — to urge the states’ senators to vote against the bill. Mendell noted that Portman has been a champion on substance-use treatment for years, and it was difficult to run ads targeting him.

“His people need to understand that this has to be a no vote,” Mendell said.

Specific constituencies aside, some policy experts regard the Senate’s plan as a wholesale reversal of the government’s path to offer health insurance to ever-wider groups of Americans, piece by piece. That started with the creation of Medicaid and Medicare as part of President Lyndon B. Johnson’s Great Society and could be ending with the ACA.

“This is bringing us back to where we were before 1965,” said Paul Starr, a Princeton University professor of sociology and public affairs who has written extensively about the history of U.S. health-care policy. “There is no longer the federal commitment to back up the states in terms of health care for the poor.”